1. Field of the Invention
This invention relates to a crosslinked poly(vinyl acetal) composition and a process for cross-linking poly(vinyl acetal) resins.
This invention also relates to photothermographic imaging elements and to the protection of the element from surface deformation during thermal development by crosslinking a poly(vinyl acetal) binder layer in the element.
2. Background of the Art
Silver halide photothermographic imaging materials, often referred to as `dry silver` compositions because no liquid development is necessary to produce the final image, have been known in the art for many years. These imaging materials basically comprise a light insensitive, reducible silver source, a light sensitive material which generates silver when irradiated, and a reducing agent for the silver source. The light sensitive material is generally photographic silver halide which must be in catalytic proximity to the light insensitive silver source. Catalytic proximity is an intimate physical association of these two materials so that when silver specks or nuclei are generated by the irradiation or light exposure of the photographic silver halide, those nuclei are able to catalyze the reduction of the silver source by the reducing agent. It has been long understood that silver is a catalyst for the reduction of silver ions and the silver-generating light sensitive silver halide catalyst progenitor may be placed into catalytic proximity with the silver source in a number of different fashions, such as partial metathesis of the silver source with a halogen-containing source (e.g., U.S. Pat. No. 3,457,075), coprecipitation of the silver halide and silver source material (e.g., U.S. Pat. No. 3,839,049), and any other method which intimately associates the silver halide and the silver source.
The silver source used in this area of technology is a material which contains silver ions. The earliest and still preferred source comprises silver salts of long chain carboxylic acids, usually of from 10 to 30 carbon atoms. The silver salt of behenic acid or mixtures of acids of like molecular weight have been primarily used. Salts of other organic acids or other organic materials such as silver imidazolates have been proposed, and British Pat. No. 1,110,046 discloses the use of complexes of inorganic or organic silver salts as image source materials.
The preferred binding medium for silver source comprises poly(vinyl acetal) resins, particularly poly(vinyl butyral). That class of resins is highly compatible with the silver source photothermographic emulsions and does not adversely affect the sensitometric properties of the element. Poly(vinyl acetals) are fairly soft resins and, being thermoplastic, soften more when heated. During the thermal development step, the poly(vinyl acetal) binder layer is subjected to heat and pressure which often causes the binder layer to flow. This can cause distortion or embossing of the surface characteristics of the element or even discontinuities in the imaging layer and, therefore, the image itself.
Attempts have been made to toughen or crosslink the poly(vinyl butyral) layers or provide more thermally resistant top coats. The most successfully practiced method of hardening the poly(vinyl acetal) binder layer has been the addition of crosslinking agents, and, in particular, polymethylenepolyphenylisocyanate (hereinafter PPPI). However, even though such compounds may toughen the layer and reduce its deformation under heat and pressure, they often have other adverse effects. PPPI, for example, even though it is the most common crosslinking agents for poly(vinyl butyral) layers in photothermographic emulsions causes coating problems and adversely affects a broad range of sensitometric propeties. PPPI must be added to the coating solution as late in the coating formulations process and as near to the coating step as possible. It tends to quickly crosslink the poly(vinyl acetal), change the viscosity of the solution, and clog up the coating equipment if not coated immediately. This makes both large batch operations and continuous coating operations difficult. More importantly, PPPI tends to react with functional ingredients in the emulsion and change the sensitometry of the element.